While looking at the STL's separately compiled machinery for modules, I noticed that inc/ymath.h was declaring a bunch of stuff that isn't needed by any other header in inc. We can move this to src/xmath.hpp (which is being included by everything in src that needs it) without disrupting users or affecting ABI. (That is, the exported machinery in src will still be exported; there's no reason to declare it as imported in inc if we aren't going to use it there.)

This is theoretically an extremely slight throughput improvement (fewer things being declared), but realistically it just makes the codebase easier to understand - fewer spurious connections between inc and src.

I'm changing one thing while moving this from inc to src: changing _CRTIMP2_PURE_IMPORT to _CRTIMP2_PURE. Within src, they are exactly identical (and we don't use _CRTIMP2_PURE_IMPORT except for _Yarn - that one is trying to match the header stylistically, although it could be changed).

#ifndef _CRTIMP2
    #if defined CRTDLL2 && defined _CRTBLD
        #define _CRTIMP2 __declspec(dllexport)
    #else
        #define _CRTIMP2
    #endif
#endif

The only difference between _CRTIMP2_PURE_IMPORT and _CRTIMP2_PURE is when building user code using the STL as a DLL (/MD and /MDd), in which case the former expands to __declspec(dllimport). That's impossible for stl/src, and we don't need to match any other declarations, so we should use plain _CRTIMP2_PURE like everything else.

We have export validation in the internal build, but I also verified this manually.

First, notice that _Feraise wasn't marked _CRTIMP2_PURE, so it wasn't being imported/exported at all! With main, it doesn't appear in the export surface:

D:\GitHub\STL\out\build\x64\out\bin\amd64>dumpbin /exports msvcp140_oss.dll | grep -P _Feraise

D:\GitHub\STL\out\build\x64\out\bin\amd64>

With this PR, everything else is still being exported:

D:\GitHub\STL\out\build\x64\out\bin\amd64>dumpbin /exports msvcp140_oss.dll | grep -P "\b(_Dtest|_FDtest|_Denorm|_Hugeval|_Inf|_Nan|_Snan|_FDenorm|_FInf|_FNan|_FSnan|_LDenorm|_LInf|_LNan|_LSnan)\b"
       1371  55A 00082198 _Denorm = _Denorm
       1372  55B 00046670 _Dtest = _Dtest
       1376  55F 00082108 _FDenorm = _FDenorm
       1377  560 00046EB0 _FDtest = _FDtest
       1379  562 000820E8 _FInf = _FInf
       1380  563 000820F8 _FNan = _FNan
       1382  565 00082118 _FSnan = _FSnan
       1391  56E 00082178 _Hugeval = _Hugeval
       1392  56F 000821A8 _Inf = _Inf
       1394  571 00082148 _LDenorm = _LDenorm
       1397  574 00082128 _LInf = _LInf
       1398  575 00082138 _LNan = _LNan
       1400  577 00082158 _LSnan = _LSnan
       1423  58E 00082168 _Nan = _Nan
       1433  598 00082188 _Snan = _Snan